In the semiconductor industry, there is a continuing effort to increase device density by scaling device size. State of the art devices currently have device features with a dimension well below 1 micron (submicron). To form these features, a photosensitive layer is formed on a substrate or device layer, and is exposed to radiation through a reticle. The reticle typically comprises a substantially transparent base material with an opaque layer having the desired pattern formed thereon, as is well known. At the submicron level, diffraction effects become significant, resulting in exposure of portions of the photoresist layer underlying the opaque layer near the edges of features.
To minimize effects of diffraction, phase-shifted reticles have been used in the prior art. Typically, a phase-shifted reticle has an opening in the opaque layer corresponding to the pattern to be formed. Phase-shifters, which transmit the exposing radiation and shift the phase of the radiation approximately 180.degree. relative to the openings, lie along or near the outer edges of the features. The radiation transmitted through the phase-shifter destructively interferes with radiation from the opening, thereby reducing the intensity of radiation incident on the photoresist surface underlying the opaque layer near a feature edge.
Prior art phase-shifted reticles have a number of problems which limit their ability to be used to pattern some features, however. Often, it is desired to place two features in close proximity to one another. For example, contact or via openings may be placed in a closely spaced array. In :prior art phase-shifted reticles, each opening in the array has a phase-shifting rim surrounding it. Since the openings are closely spaced, the phase-shifting rims of two openings may be very close to or in contact with one another. In this case, the phase-shifting rims of the two close opening patterns are roughly equivalent to one very wide rim. Unfortunately, as the phase-shifting rim width is increased, the intensity of radiation underneath the phase-shifting rims increases. The increased intensity causes a deep recession in the developed photoresist layer, and may in fact cause a portion of the photoresist to be removed between two openings. This phenomenon is know as the proximity effect, and it occurs if two phase-shifting rims are positioned at approximately 0.55 .lambda./NA or less, where X is the wavelength of the exposing radiation, and NA is the numerical aperture of the lens of the lithographic printer being used.
To overcome this problem, an inverted phase-shifted reticle has been discovered. The inverted phase shifted is described in the above-referenced copending U.S. patent application Ser. No. 07/933,400, which application is assigned to the assignee of the present invention, and which application is hereby incorporated by reference. Several methods of manufacturing the inverted phase-shifted reticle are described in the above-referenced copending U.S. patent application Ser. No. 07/933,341, which application is assigned to the assignee of the present invention, and which application is hereby incorporated by reference. In the inverted phase-shifted reticle, one phase shifted feature is inverted with respect to a proximate phase-shifted feature, such that two adjacent phase-shifters are 180.degree. out of phase with respect to one another. Thus, the improved resolution of phase-shifting is achieved, while at the same time, the proximity effect is greatly reduced or eliminated. An example of the inverted phase-shifted reticle is shown in FIG. 2.
Another phase-shifting approach is the attenuated phase-shifting mask (APSM). Some of the problems of prior art APSMs, and an improved attenuated phase-shifted reticle, are disclosed in co-pending U.S. patent application Ser. No. 07/952,061, filed Sep. 25, 1992, which application is assigned to the assignee of the present invention, and which application is hereby incorporated by reference. One of the problems of prior art APSMs which is solved by the reticle disclosed in U.S. patent application Ser. No. 07/952,061, is focal shift in opposite directions.
It has recently been discovered that the problem of a focal shift in opposite directions can occur in certain inverted phase-shifted reticles. The phenomenon of focal shift is explained in reference to FIG. 1A, and the problem of a focal shift in opposite directions is explained in reference to FIG. 1B.
In the following FIGS. 1A and 1B, a graphical representation of defocus versus critical dimension of an opening in a positive photoresist layer is shown. It will be understood that the actual values can vary considerably based upon the feature being formed, exposure parameters, including time and energy of the exposure, printer parameters and other factors. The FIGS. 1A and 1B provide an example for one set of exposure and printer parameters. Referring to FIG. 1A, a graph of critical dimension (CD) in a photoresist layer plotted against defocus (distance between photoresist layer and best or perfect focus) is shown. As can be seen from curve 10 of FIG. 1A, if the image is defocused in either the positive or negative direction, the dimension of an opening in the resist decreases. The decrease in CD is due to the fact that the intensity of the exposing radiation decreases with either positive or negative displacement from perfect focus. If the process specification allows for a CD in the range of 0.3-0.5 microns, then for the example shown in FIG. 1A, the defocus can be in the range of approximately -0.75 .mu. through + 0.75 .mu., since the CD varies from about 0.31 .mu. at -0.75 defocus, to 0.4 .mu.at 0 .mu. defocus, and back down to 0.3 .mu. at +0.75 g defocus. Outside of this range, the CD falls below 0.3 .mu. and is outside of the specified range. The range between -0.75 .mu. and +0.75 .mu. is the depth of field (DOF), and is shown by the line 11. Thus, so long as the photoresist layer is within this DOF, the CD will be within specification. As is well known, a large DOF is desirable, as the wafer topography and other factors cause the level of the photoresist layer to vary considerably across the exposure field of the printer.
In a phase-shifted reticle, the above-mentioned focal shift occurs, whereby curve 10 shifts as the phase difference between the phase-shifter and the feature varies from 180.degree. (phase error). As described in the above-referenced application Ser. No. 07/952,061, the direction of focal shift depends upon whether the phase-difference between the shifter and the feature is greater or less than 180.degree. (i.e., whether there is positive or negative phase error. For example, for a given feature/shifter relationship, if the phase difference is less than 180.degree., the curve 10 shifts to the right. If the phase difference is greater than 180.degree., the curve 10 shifts to the left. Referring to FIG. 1B, curve 12 shows a plot of CD versus defocus for a feature where the phase difference between the feature and its phase-shifter is less than 180.degree., and curve 13 shows CD versus defocus for a feature where the phase difference between the feature and its phase-shifter is greater than 180.degree.. The shape of the CD versus defocus curve does not change significantly due to phase error. Rather, the main effect of phase-error is to cause the curve to shift.
The problem caused by focal shift in opposite directions is illustrated in FIG. 1B. Focal shift in opposite directions as used herein means that a single reticle has features for which the curve 10 of FIG. 1A has shifted right and features for which it has shifted left That is, a reticle with both negative and positive phase-error. On such a reticle, the DOF will be greatly decreased due to the focal shift in opposite directions. As can be seen from FIG. 1B, for those features for which the curve has shifted right (curve 12), the CD will be below 0.3 .mu. if the defocus is approximately 0.5 .mu. or greater, while those features for which the curve has shifted left (curve 13) will have a CD below 0.3 .mu. if the defocus is approximately -0.5 .mu. or less. Thus, DOF 14 now extends only from -0.5 .mu. through +0.5 .mu. due to the focal shift in opposite directions. This compares to DOF 11 of -0.75 .mu. through +0.75 .mu. for a reticle with no focal shift or focal shift in one direction only.
What is needed is a phase-shifted reticle allowing for patterns having a phase-shifting element to be placed closely together without causing an unacceptable increase in exposure intensity between the patterns, and without having focal shift in the opposite direction from one another.